neptunium(Np), radioactive chemical element of the actinoid series of the periodic table, first transuranium element to be artificially produced, atomic number 93. Though traces of neptunium have subsequently been found in nature, where it is not primeval but produced by neutron-induced transmutation reactions in uranium ores, Edwin M. McMillan and Philip H. Abelson first found neptunium in 1940 after uranium had been bombarded by neutrons from the cyclotron at Berkeley, Calif. Neptunium has been produced in weighable amounts in breeder reactors as a by-product of plutonium production from uranium-238 (about one part neptunium is produced for every 1,000 parts plutonium). All neptunium isotopes are radioactive; the stablest is neptunium-237, with a half-life of 2,140,000 years, and among the most unstable is neptunium-232, with a half-life of 13 minutes.

Neptunium, a silvery metal, exists in three crystalline modifications; the room-temperature form (alpha) is orthorhombic. Neptunium is chemically reactive and similar to uranium with oxidation states from +3 to +6. Neptunium ions in aqueous solution possess characteristic colours: Np3+, pale purple; Np4+, pale yellow-green; NpO2+, green-blue; NpO22+, varying from colourless to pink or yellow-green, depending on the anion present.